These machines for stacking flat items, which are used in many industrial sectors, apart from stacking batches made up of a determined number of such items from a constant and continuous flow thereof, tend to count and extract those batches. Flat items of this type can be aluminum sheets, paper, chipboard, plywood, cardboard etc.
In the industry for transforming and manipulating cardboard and particularly corrugated cardboard, there is a range of so-called “FLEXO-FOLDER-GLUER LINES” machines, known by the abbreviation “FFG LINES”, which are used for manufacturing printed and die-cut cardboard sheets, as well as for forming folded, glued and/or stitched boxes from the previous cardboard sheets. Square or rectangular cardboard sheets are introduced in this type of machine and bundles made up of a determined number of boxes (for example from 2 to 30) stacked one on top of the other are obtained at the end of the line. The height of these bundles can vary between limits generally comprised, for example, between 5 and 350 mm. The purpose of this extraction is to form the bundles to send them to a subsequent strapping and packing process, finishing the bundle in a transport pallet.
These lines in which the transformation of the cardboard sheets is carried out comprise different modules, in which different operations are carried out. These modules are basically the following:
INTRODUCER: It is the module responsible for feeding the sheets to the line. It feeds a sheet by advancement of the printing roller which is in the printer module.
PRINTER: It is the module responsible for printing the sheets with ink.
SLOTTER: It is the module responsible for cutting the slots, marking the folding slits and the gluing flap.
CUTTER: It is the module responsible for carrying out all the other irregular cuts which the SLOTTER cannot carry out, when the cardboard sheet optionally requires so.
FOLDER: It is the module responsible for gluing the flap and folding the panels of the box on the previously marked slits, thus forming the box.
STITCHER: It is the module responsible for stitching the flap of the box with staples.
STACKER: It is the module responsible for stacking said sheets or boxes in perfectly counted and arranged bundles.
Conventionally, the manufacturers of these “FFG” LINES use a stacking module at the end of their lines to generally carry out a process which is fairly problematic for all manufacturers today, and the basic operations of which are: receiving, aligning and squaring up the sheets or boxes that come from the folder, because they may be slightly out of square; forming each bundle with the exact number of boxes and separating the last box of a bundle and the first box of the next bundle; this process has to be carried out in a fraction of a second; stacking the boxes in piles or stacks without said boxes coming from the folder being able to open, i.e., preventing the panels of the boxes from being unfolded during the stacking process and jams from occurring in the operation of the stacker; and removing the bundles or stacks of boxes from this module and introducing them into the next, which generally tends to be a strapping machine. It thus proceeds to the strapping of the bundle.
The existing conventional stackers can carry out the stacking in two ways, namely, by the lower part, i.e., the box enters the stack below the box which had previously entered, or by the upper part, in which case the box enters from the top, one on top of the other.
Document EP-A-0666234 describes a station for stacking, separating and evacuating the batches to the discharge end of a machine for transforming cardboard sheets, wherein the station stacks folded and flattened cardboard boxes in the lower area thereof, comprising element intake means, these elements falling on a stack which is formed on a raised table which descends as the stack is created, the upper part of the table being formed by rollers or treadmills, also comprising separator arms joined to a mobile horizontal crossbar which is displaced parallel and perpendicular to the plane of the table, the separators being positioned to receive the plate elements. It also comprises a discharge conveyor, to the level of which the table descends for evacuating the batch or elements bundle.
Document EP-A-0006771 describes a process and a device for stacking sheets, based on a system of conveyor belts which displace the cardboard boxes and deposit them in a stack with height-adjustable base, such that that when a determined height is reached, the stacker interrupts the loading of boxes to the stack.
Document EP-A-0578990 in turn describes a sheet retaining member for storing the stack, this retaining member being formed based on elastic bars, displaceable by pistons or cylinders, to retain the sheets of cardboard boxes when these are stacked.
Document EP-A-0529708 describes a machine having means for displacing each sheet to the infeed end through the infeed end on the upper area, having rotating elastic cams by means of which compacting and flattening of the folded boxes is carried out, introducing them into the inside until reaching a stop. In this machine, and after the operations previously mentioned, the folded boxes are then lowered to a stacking area and, when the stack is of a determined height, the entire assembly or bundle is displaced due to the action of rollers.
The Spanish patents with numbers ES-512711, ES-523290, ES-523291 and ES-523.292, which correspond to the U.S. Pat. No. 4,500,243, describe improvements in machines or apparatus for feeding successively synchronized sheets, based on a corrugated cardboard sheet feeder, synchronized with other adjacent machines, using negative atmospheric pressure to fasten each sheet against the transporting means made up of conveyors, all without the need for valves and without interrupting the suction pressure. Likewise, a mechanism for feeding, with stopping and omission, which allows the feeding of sheets in alternate cycles and by selective stopping is described in these Spanish patents.
On the other hand, U.S. Pat. No. 5,980,196 describes a box counter-ejector which feeds a machine in which means for stacking the folded cardboard boxes are established. These means have pressure elements which keep the box folded during the displacement thereof along the conveyor belts, from the infeed end area to the stacking area. Fingers which are always introduced at a determined height between the boxes are also described in the United States patent, dividing the stack bundle so that the bundle has a height selected by the lower area itself of the fingers at the discharge end of the machine such that the stacked boxes arranged on these fingers form what will be the following bundle.
The staking machines must carry out the stacking and counting of the flat items and separating and extracting the corresponding batches of flat items in a greatly reduced time lapse, and at the same time they must avoid mistakes in counting, jams and flaws in the flat items; therefore its good operation is critical in the production lines of flat items because in the event of any failure such as a jam, for example, the entire production line is paralyzed. However, the stacking machines of the state of the art can still be improved with regard to the combination of a suitably fast work speed and a very high operational safety.